1. Field of the Invention
The present invention relates to improved processes for enhancing solid/liquid separation of solid-stabilized emulsions formed within hydrometallurgical solvent extraction circuits. More specifically, the present invention relates to the use of water-soluble or water swellable polymeric aggregating agents for recovering organic solvent extractant from solid-stabilized emulsions formed in the hydrometallurgical solvent extraction circuits.
2. Technical Background and Description of the Related Art
Liquid-liquid extraction technology is commonly employed to recover and concentrate metal values from aqueous leach liquors that have been used to dissolve such metal values from their ores. In such a procedure, an aqueous leach liquor containing metal values in ionic form is thoroughly admixed in a mixing device or extraction column with an organic solvent containing an ion exchange or metal complexing agent. In admixture with the metal ion-containing aqueous leach liquor, the organic solvent medium selectively or preferentially dissolves and thereby extracts the metal values from the aqueous leach liquor. After organic solvent extraction of metal values has occurred to the extent desired, the organic/aqueous mixture or dispersion is fed to a settling tank or to the settling region of a mixer-settler wherein the metal-laden organic solvent extract separates by gravity from the metal-depleted aqueous solution. Metal values can be subsequently recovered from the metal-laden organic solvent extract by, for example, conventional electrowinning procedures.
When such an extraction process is conducted in a continuous manner, it is common to have a third phase liquid layer of impurities form between the organic and aqueous phases, i.e., at the organic/aqueous interface in the settling tank or region. Such a third phase impurities layer is colloquially referred to as “gunk” or “crud” in the hydrometallurgical industry. The “crud” layer consists primarily of the organic extraction solvent which contains smaller amounts of aqueous liquid and undissolved particulate material. Such a third phase “crud” layer which forms in the settling tank or region is actually a dispersion, e.g., an emulsion or suspension, of the aqueous and particulate matter in an organic matrix or continuous phase.
During continuous liquid-liquid extraction operations, the third phase layer of impurities often accumulates at the organic/aqueous interface. This “crud” or “solid-stabilized emulsion” may also collect in other regions of the separation device in addition to the oil-water interface. Accumulation is such that in conventional extraction operations, the settling tank itself must be shut down, dumped and flushed out from time to time in order to prevent the accumulating layer of impurities from eventually interfering with the efficiency of the extraction procedure. Crud impedes complete organic solvent-aqueous separation anywhere in the SX process. Shut down of the settling tank or mixer-settler in the extraction process is, of course, economically disadvantageous. Thus, crud formation is a persistent problem common to all solvent extraction operations and is considered one of the primary issues and most common complaints.
Since the crud is at least about 50% organic solvent, it is economically important to recover most if not all of the entrained organic phase. Due to the nature of the problem and the cost of the organic reagent, various systems, processes, and/or equipment for the removal or prevention of crud have been developed as detailed by at least the following patents: U.S. Pat. No. 4,126,551; U.S. Pat. No. 4,190,633; U.S. Pat. No. 4,207,302; U.S. Pat. No. 4,290,882; U.S. Pat. No. 4,759,913; U.S. Pat. No. 5,024,821; U.S. Pat. No. 5,334,317; U.S. Pat. No. 5,334,317; U.S. Pat. No. 5,758,255; U.S. Pat. No. 5,772,730; U.S. Pat. No. 5,997,732; U.S. Pat. No. 6,261,341; U.S. Pat. No. 6,419,619; U.S. Pat. No. 6,500,232; U.S. Pat. No. 7,338,608; U.S. Pat. No. 7,381,332; and U.S. Pat. No. 7,780,854. However, few of these solutions are actually practiced in the field as most suffer from being labor intensive, i.e., involve the physical removal of the crud by an operator, involve long processing times, and/or require costly plant modifications or equipment. The chemical means of preventing crud formation have also not been widely successful. Such means typically involve the use of substantial amounts of chemicals to treat the entire feed stream. The chemicals can then have negative effects on the downstream process (solvent extraction performance), or simply be uneconomic/inefficient.
Accordingly, processes for enhancing solid/liquid separation within hydrometallurgical solvent extraction systems require further improvement. Processes that lead to improved processing of crud by increasing recovery of the organic reagent and/or reducing crud processing time, thereby effectively reducing mining costs for such metals, would be a useful advance in the art and could find rapid acceptance in the metallurgical mining industry.